This invention relates to color wheels for projection systems and to a method for producing color wheels.
The invention pertains to a color wheel of the type employed in generating sequential color images using projection equipment. Such a color wheel can be produced by appropriately coating and configuring a disk-shaped glass substrate. It is especially the configuration process, however, that is expensive. In practice, therefore, a color wheel is in most cases composed of circular sections. It is typically produced by separating ring sectors or circular sections from dichroic plates and attaching them on a rotary disk in such fashion that the outer regions of the circular sections protrude past the rim of the disk and the outer edges of the circular sections form an annular circle. The color wheel thus produced, when to be used, is positioned in the optical path of an illuminating beam which, depending on the optical characteristics of the circular section currently in the optical path, will be transmitted or reflected for instance as a function of the wavelength concerned. Rotating the wheel makes it possible to generate a sequence of different colors.
This invention also relates to a method for producing the circular sections for use in a color wheel. For a more comprehensible description of this invention the following will identify and define some of the geometric parameters of the circular sections with reference to FIG. 1. A standard circular section 3 is circumscribed by an outer annular edge 5, an inner annular edge 7 as well as two radial edges 9, 9′ that enclose the central angle φ and extend in a direction perpendicular to the outer and inner annular edges 5, 7. For the purposes of this invention the outer annular edge 5 is divided into a central circular arc section 11 and two lateral circular arc sections 13 and 13′ in such fashion that the chord to the central circular arc section 11 is of the same length as the chord to the inner annular edge 7 and that the two lateral circular arc sections 13, 13′ are of an identical length.
It is not generally possible to completely and fully cover a two-dimensional area with identical circular arc sections. As an inevitable result, the separation of such circular sections from plate material cannot make full use of all the material, i.e. parts thereof are wasted and the material yield is less than 100%. The plate material is usually preprocessed involving expensive procedures. For example, the plates used in making color wheels are first vacuum-coated. Maximum utilization of the material is therefore desirable. Another factor that further reduces material utilization in the separation process and that depends on the method by which the plate material is separated into circular sections, is the need for the individual circular sections to be more or less spaced apart so that clean, reproducible edges can be obtained. A widely used method for separating substrates from glass plates involves a scoring and breaking process (the S&B method). The surface of the plate material is first scored along the line of the intended edge and the glass is then broken along that line. There must be enough material on both sides of the scored line to permit the application of the force necessary for breaking it and making sure that the break actually follows the line of the intended edge. In that context, enough material means at least three times the thickness of the glass to be broken. If the glass is 1 mm thick, the circular arc sections must consequently be spaced at least 3 mm from the respectively neighboring circular arc section over its entire circumference, unless the circular sections share a common break line.
Accordingly, in order to increase the yield, an attempt is made to position the substrates that are to be cut out by appropriate separation on the plate material from which they are to be obtained, in such fashion that as many joint break lines as possible are produced. That, of course, depends in large measure on the intended geometric shape of the separated sections. For identical circular arc
sections the only possibility is to position a radial edge of a first circular arc section so as to coincide with a radial edge of another circular arc section. If the curvatures of circular arc sections are aligned in matching fashion, additional circular sections can be positioned along the free radial edges, provided they are not more or less completely closed circular arc sections and provided the size of the plate permits it. The circular sections thus form an undulating, serpentine strip. This method is therefore referred to below as the serpentine method.
Another possibility to produce joint cutting edges and thus to achieve a better yield is to use a modified circular section, assuming the application permits it. As an example in the case of the color wheel described, the inside center area of the circular sections is used for the mechanical mounting only, thus allowing the inner annular edge to be varied as necessary.
A desirably modified circular section features an inner annular edge whose curvature matches the outer annular edge. In that case the circular sections can be so positioned that the inner annular edge 7 of a circular section constitutes a common edge break line with the central section 11 of the outer annular edge 5 of another circular section. The circular sections will thus form columns in which they are positioned one above the other. This process is therefore referred to below as the columnar method.
Applying the columnar method by which the inner annular edge is adapted, only a small part of the perimeter shares a joint break line intended to minimize material loss. For the remainder of the perimeter, at least three times the distance to the break line must be allowed to permit proper breaking. Since in most cases the plate material used for the segmentation is preprocessed by a preceding, cost-intensive coating procedure, this loss of material is still a significant factor in terms of the manufacturing cost.
It is the objective of this invention to introduce a solution that overcomes the shortcomings of prior art.
Simply combining the serpentine method and the columnar method will not work: The technician who tries to apply the serpentine method by moving the undulations closer together will fail due to the fact that parts of the outer edges of the mutually adjoining circular sections have a differently oriented curvature, so that a common break line is not possible even in the case of a relatively unencumbered configuration of the inner annular edge.
The technician who, by applying the columnar method along the concept of the serpentine method, tries to align the radial break lines of one column with the radial break lines of another column will fail due to the fact that the circular sections would overlap.